Digging diversion swale away from house that overflows into series of rain gardens. Begins near the drip line of old oak tree. Wondering if it is harmful to leave big roots exposed. Any guidelines on pruning roots, care, tools, local people I should talk to, books to read, etc. would be much appreciated!
A lot of that has to do with the type of oak, climate, and that oaks particular situation. Generally, disturbing a trees roots will negatively impact the tree, but what percent of roots are you disturbing? How deep? Was the tree already fighting disease? With weather pattern changes and many forests losing lots of trees, small things can be a big deal for some trees and have no visible impact on others.
Sorry for the complicated answer. A good way to try and observe the potential impacts is to look at the health of similar oaks in the area. Are they all healthy and thriving with lots of young seedlings coming up? How about your tree? Fully healthy looking, low on bore marks and dead branches? See whether other oaks of the same type in your area have root exposure similar to what you proposed. Then see if there are any other things to consider... Like competition from another tree or whether those roots happened to be the ones watering it from a spring in a dry climate, etc.